The coating of electrically conductive substrates by electrodeposition is a well known and important industrial process. (For instance, electrodeposition is widely used in the automotive industry to apply primers to automotive substrates). In this process, a conductive article is immersed as one electrode in a coating composition made from an aqueous emulsion of film-forming polymer. An electric current is passed between the article and a counter-electrode in electrical contact with the aqueous emulsion, until a desired coating is produced on the article. The article to be coated is the cathode in the electrical circuit with the counter-electrode being the anode.
Resin compositions used in cathodic electrodeposition baths are also well known in the art. These resins are typically manufactured from polyepoxide resins which have been chain extended and adducted to include a nitrogen. The nitrogen is typically introduced through reaction with an amine compound. Typically these resins are blended with a crosslinking agent and then neutralized with an acid to form a water emulsion which is usually referred to as a principal emulsion.
The principal emulsion is combined with a pigment paste, coalescent solvents, water, and other additives (usually at the coating site) to form the electrodeposition bath. The electrodeposition bath is placed in an insulated tank containing the anode. The article to be coated is made the cathode and is passed through the tank containing the electrodeposition bath. The thickness of the coating is a function of the bath characteristics, the electrical operating characteristics, the immersion time, and so forth.
The coated object is removed from the bath after a set amount of time. The object is rinsed with deionized water and the coating is cured typically in an oven at sufficient temperature to produce crosslinking.
The prior art of cathodic electrodepositable resin compositions, coating baths, and cathodic electrodeposition processes are disclosed in U.S. Pat. Nos. 3,922,253; 4,419,467; 4,137,140; and 4,468,307, which are incorporated herein by reference.
Three very important characteristics of an electrocoat system are its nonyellowing characteristics, its bake off losses, and smoothness. Nonyellowing is important since typically an electrocoat will be covered with top coats (i.e. monocoat or base coat/clear coat). Current electrocoat systems cause yellowing of light colored topcoats. This is thought to be caused by the use of toulene diisocyanate (TDI) as part of the crosslinker.
Bake off loss means the loss of resin and solvents during the baking of the coating composition. A reduction in bake off loss is desirable because it results in less emissions to the environment, less oven fouling, and substantial cost savings since more coating composition is retained on the coated substrate. Current electrocoat systems have typical bake off loss of about 14 to 16 percent.
It is very important that the electrodeposited layer be of high quality even though it typically will be covered with top coats. Defects in the electrodeposited layer such as cratering or roughness may telegraph through the top coats. Therefore, it is necessary that the electrocoat primer be smooth.